The present invention generally relates to a coaxial resonator and more particularly, to a dielectric material coaxial resonator mainly employed for filter elements and the like of an electrical filter device.
A conventional dielectric coaxial resonator of the above described type is shown in FIG. 1. This resonator intended to be used for a filter element of a 1/4 wavelength two stage electrical filter device, and includes first and second single resonator units Ra and Rb of the same construction aligned laterally side by side so as to be molded into one unit.
Each resonator unit Ra, Rb includes a respective dielectric material block Da, Db formed of a ceramic material or the like in the shape of a rectangular parrallelepiped having a through-bore Ha, of a circular cross section formed at central portions thereof. Inner conductive layers or inner conductors Ea, Eb, respective are formed over the inner peripheral surfaces of the through-holes Ha, Hb. An outer conductive layer or outer conductor Ec is formed over the outer peripheral surfaces of the dielectric material blocks Da, Db, and an electrode Es is formed on one end face i.e. lower end face F1 in FIG. 1 to short-circuit the outer conductor Ec with the inner conductors Ea and Eb. The other end face i.e. upper end face F2 in FIG. 1 is formed as an open end face so that the outer faces of the dielectric material blocks Da and Db are exposed. In the actual structure, the dielectric material blocks Da and Db for the first and second single resonator units Ra and Rb are molded in the form of one block, with a coupling degree adjusting bore V, for example, of rectangular cross section, being formed at a central portion between said resonator units Ra and Rb. Moreover, in the through-bores Ha and Hb of the resonator units Ra and Rb, electrically insulative bushings Ba and Bb in which input and output pin terminals Pa and Pb are fitted under pressure, are respectively inserted. These bushings Ba and Bb serve to support the pin terminals Pa and Pb, and also to achieve a coupling electrostatic capacity (referred to merely as a coupling capacity hereinbelow) between the inner conductors Ea and Eb, and the pin terminals Pa and Pb.
In the conventional resonator having the construction described above, when a high frequency signal is applied, for example, to the pin terminal Pa of the first resonator unit Ra, said signal is applied from the inner conductor Ea to the first resonator unit Ra through the coupling capacity produced between the pin terminal Pa and the first inner conductor Ea. Subsequently, the above signal is propagated to the second resonator unit Rb which is magnetically coupled with the first resonator unit Ra through the coupling degree adjusting hole V, and is then fed to the second pin terminal, i.e. output side pin terminal Pb from the inner conductor Eb through the coupling capacity produced between the second inner conductor Eb and second pin terminal Pb. The known resonator is accommodated in a metallic casing (not shown) through a spring means (not shown) in an electrically conducted state so as to function as a filter device.
However, in the conventional coaxial resonator as described above, it is impossible to mount the single resonator units Ra and Rb themselves directly onto a printed substrate or printed circuit board of electronic equipment, thus requiring the use of the input and output pin terminals Pa and Pb and bushings Ba and Bb, etc. for this purpose, while in the case where the resonator is used as a filter device, the metallic casing for accommodating the resonator therein and spring members, etc. are separately required, inevitably resulting in an increase in the number of parts involved and complication of assembling work and making it impossible to achieve reduction in the material cost and manufacturing cost. Such disadvantages as described above become more conspicuous especially as the number of stages of a multi-stage type filter is increased.
Furthermore, the bushings Ba and Bb are normally molded from a synthetic resin material which cannot normally withstand high temperatures, and are not reliable in that the material becomes unstable in its characteristics or is subjected to fatigue or breakage in a short period under circumstances where temperature variation is excessive.
The problems as explained above are not limited to the multi-stage type coaxial resonators represented by the two stage type coaxial resonator described above, but similarly occur in an arrangement including only a single resonator unit.